Mineral analysis systems, such as the QEMSCAN® (Quantitative Evaluation of Minerals by Scanning electron microscopy) and MLA (Mineral Liberation Analyzer) from FEI Company, the assignee of the present invention, have been used for many years to determine the presence of valuable minerals in mines. Such systems direct an electron beam toward the sample and measure the energy of x-rays coming from the material in response to the electron beam. One such process is called “energy dispersive x-ray analysis” or “EDS,” which can be used for elemental analysis or chemical characterization of a sample. Determining the elements present in a mineral sample is referred to as “elemental decomposition.”
EDS systems rely on the emission of x-rays from a sample to perform elemental analysis. Each element has a unique atomic structure, so x-rays that are characteristic of an element's atomic structure are unique to that element. To stimulate the emission of x-rays from a sample, a beam of charged particles is focused onto the sample, which causes electrons from inner shells to be ejected. Electrons from outer shells seek to fill this electron void, and the difference in energy between the higher energy shell and the lower energy shell is released as an x-ray, which can be detected by an EDS detector.
By measuring the number and energy of the x-rays emitted from a specimen using an energy-dispersive spectrometer and comparing the measured spectra to a library of reference spectra of known compositions, the unknown elemental composition of the specimen can be determined. EDS analysis, especially when coupled with back-scattered electron (BSE) analysis, can be used to quantify a wide range of mineral characteristics, such as mineral abundance, grain size, and liberation, that is, how easy it is to separate a desired mineral from background materials. Existing EDS analysis systems include QEM*SEM technology, which is assigned to FEI Company, Inc, the assignee of the present invention.
A mineral classification system must be capable of comparing each unknown measured spectrum to a library of known mineral spectrums, and then making a selection based on which known mineral is most similar to the measured spectrum. Typically, to find the most similar spectrum requires the use of a metric that represents the degree of similarity between the measured data and the known material.
Mineral analysis systems of this type are also used in the oil and gas industry. Drill cuttings (drill bit-induced rock chips) and diamond drill cores can be analyzed to allow geologists to determine the exact nature of the material encountered during drilling, which in turn allows more accurate predictions as to the material still ahead of the drill, thus reducing risk in exploration and production. During drilling, a liquid referred to as “mud” is injected into the well to lubricate the drill and return the cuttings out of the well. A sample can be taken from the mud that includes cuttings from the drill. Documenting cuttings and cores as accurately as possible, both at the time of drilling and post-drilling, is an important facet of analyzing the drilling process. The information obtained from cutting and coring samples allows characterization of down-hole lithological variation in a reservoir sequence, a critical requirement in exploration and production wells, and mineralogical and petrographic studies underpin the fundamental understanding of reservoir and seal characteristics. Traditional optical, scanning electron microscope (SEM), electron probe micro-analyzer (EPMA) and x-ray diffraction (XRD) analysis methods are well established and widely used within the industry.
Mineral analysis attempts to identify the minerals present and their relative proportions at any point to which the electron beam is directed. An analysis instrument typically measures an x-ray signal, determines what elements are present, and then translates that list of elements into a mineral identification using a database of mineral definitions.
A particular mineral always has peaks at certain energies in the x-ray spectrum. To accurately determine which peaks correspond to which minerals, it is necessary to calibrate the instrument being utilized before it can identify the unknown minerals. For this reason, instrument calibration techniques are particularly important for meaningful analysis.
An EDS instrument is typically calibrated using either a standards-based calibration or a standards-less calibration. In standards-based calibration, known materials, referred to as “standards,” are analyzed and the results are used to establish a spectra library. Unknown samples are then compared to the calibration database to determine what elements are present. The spectrum of the calibration standards must be obtained under identical conditions as the conditions used to obtain the spectrum of the unknown material. Because conditions in the measuring instrument depend on a number of factors, conventional standards-based calibration requires measurement of the full set of standards each day when the machine is used. For example, the QemScan instrument described above currently identifies a maximum of 72 different elements found in mineral samples. Conventional standards-based calibration would require that each instrument have a set of 72 elemental/mineral standards to generate a full set of x-ray spectra for the decomposition. This type of calibration is prohibitively expensive, both in terms of the cost to supply a standard block containing the full set of elemental and mineral standards, and in terms of the operator time required and reduced instrument through-put. In addition, as there are several elements that only exist in minerals and can't be measured individually, it is not possible to perform this calibration for those elements.
Factory calibration using prior art methods is not practical because conventional standards-based decomposition requires that the x-ray spectra of all the elemental standards be acquired on the same machine under the same conditions as the x-ray spectrum of the mineral being analyzed. Prior art suggests that standards based elemental analysis requires that the elemental x-ray spectra need to be measured on the same machine and cannot be taken from another machine. This is because variations in the way a user sets up a particular instrument, including, for example, particular sample geometries, heights, etc., will alter the properties of the x-ray spectrum and affect the calibration.
Using standards-less EDS analysis, an analysis of a particular mineral sample is made without comparing to known standards. Based on characteristics of the spectrum collected, such as peaks and emissions at certain energies, the element list is narrowed and ultimately an element is selected. Standards-less analysis is far more complex and subject to greater inaccuracies than standards-based analysis, but it provides for easy setup for the user because it does not require calibrating the instrument using certain parameters for all possible elements.
Accordingly, what is needed is an efficient calibration method and apparatus that is less costly, simpler and easier to implement, and that allows samples to be measured much more rapidly.